What to actually say
Open with this, more or less word for word: I need to talk to you about us, and I don't want you to brace for something small, because it isn't.
I'm ending this. Not pausing it, not taking space, not stepping back — ending it.
Says the actual thing, so it can't be filed away as a rough patch by morning.
You haven't done anything wrong, and neither have I. I just don't want to be in this relationship anymore, and that's the whole truth, even though it's not a satisfying one.
Splits fault from decision so they stop hunting for the crime that explains it.
I know that's confusing to hear when nothing's 'wrong.' I don't have a tidier reason to hand you, and I'm not going to invent one.
Names the confusion out loud instead of pretending your reasoning will land cleanly.
I'm not asking you to agree with this tonight or feel okay about it. I need you to hear that it's decided.
Removes the invitation to debate whether the breakup is valid.
How it usually goes
A version of the conversation with the push-back left in, because that is the part you need to have already heard once.
What not to say
I still love you, I just don't know what I want right now.
Leaves the door open and turns a decision into an invitation to negotiate.
Maybe we should just take a break for a while.
This is the exact softening that leaves the other person unsure anything actually ended.
You deserve someone so much better than me.
Plays like self-blame and gives them something to argue you out of instead of accepting the decision.
If they ask whether there's someone else or demand a 'real' reason
Don't manufacture a cleaner reason to make it land easier — it will unravel later and cost you both something worse. Say what's true even if it's underwhelming: "There's no one else, and honestly it would almost be simpler if there were, because then you'd have something to argue with. There isn't. I just don't want this anymore." If they keep pressing for the deeper reason, you're allowed to say you've given it and you're not going to keep re-litigating your own feelings on their timeline.